The present invention relates to improvements in braking systems for automotive vehicles, and more particularly to improvements in braking systems which comprise one or more dynamic brakes (preferably hydrodynamic brakes) and one or more mechanical friction brakes.
Braking systems which comprise dynamic and mechanical brakes are equipped with means for actuating the brakes (such actuating means may comprise a pivotable brake pedal) and for selecting the magnitude of the braking force, as well as with means for delaying or retarding the action of mechanical brakes. The mode of operation is such that the braking force which is furnished by mechanical brakes is reduced in response to increasing torque which is furnished by the dynamic brake or brakes, and vice versa. The aforementioned retarding means insures that the dynamic brake or brakes are actuated prior to mechanical brakes in order to protect the mechanical brakes against unnecessary wear. Whenever the braking system is in use, the sum of braking forces furnished by the dynamic and mechanical brakes should match the desired braking force. The magnitude of desired braking force is selected by the position of the input element (e.g., the aforementioned brake pedal) of the actuating means. As a rule, the actuating means comprises a single input element whose pivoting or other displacement initiates the actuation of mechanical and/or dynamic brakes.
It is well known that, as long as the speed of a vehicle (whose braking system includes mechanical and dynamic brakes) exceeds a predetermined lower limit, the braking action of a hydrodynamic brake in a given position of the pedal remains at least substantially constant even if the speed of the vehicle decreases (provided that such speed does not fall below the aforementioned predetermined lower limit). This is due to the provision of a regulating unit which automatically limits the dynamic braking force to a desired value. Such regulation of braking force can be achieved, in the case of a hydrodynamic brake, by changing the degree of filling of the working circuit through the medium of the regulating unit. However, once the speed of the vehicle decreases below the predetermined lower limit (such lower limit is reached when the working circuit of a hydrodynamic brake is filled to capacity), the braking action of a dynamic brake decreases very rapidly (the curve representing such reduction of braking action is a parabola). This is attributable to the well-known physical laws pertaining to operation of dynamic brakes and analogous apparatus.
If the braking system comprises dynamic and mechanical brakes, the mechanical brakes should take over as soon as the braking action of the dynamic brake begins to decrease as a result of deceleration of the vehicle to a speed which is less than the aforediscussed predetermined lower limit. Such action of mechanical brakes should be effected automatically, i.e., it should not be initiated by the operator of the vehicle which embodies the braking system, and the braking action of mechanical brakes should increase proportionally with reduction of the braking force which is furnished by the dynamic brake. This is achieved by the provision of the aforementioned retarding or delaying means which normally comprises a one-way valve designed to regulate the fluid pressure for operation of mechanical brakes in dependency on the braking force of the dynamic brake as well as in dependency on momentary position of the input element of the actuating means (i.e., in dependency on the desired braking force).
If the just discussed conventional braking system is incorporated in a vehicle (e.g., a road vehicle) and comprises a single dynamic brake, the dynamic brake is normally installed in such a way that it can apply braking torque to the rear axle of the vehicle. Furthermore, and for the reasons of safety, separate circuits are provided for actuation of mechanical brakes which act upon the front and rear axles of the vehicle. As regards the cooperation between the dynamic and mechanical brakes in such braking systems, one can proceed as follows:
The aforementioned retarding means influences the mechanical brakes for the front and rear axles in dependency on the braking action of the dynamic brake. In such braking systems, it happens quite frequently that the braking force is furnished by the dynamic brake alone, as long as the desired braking force does not exceed the maximum force which can be furnished by the dynamic brake alone. For example, the braking action can be supplied by the dynamic brake alone as long as the speed of the vehicle is above the aforementioned predetermined lower limit, i.e., as long as the vehicle is driven at a medium or high speed.
Alternatively, the retarding means controls only the mechanical brake or brakes for the rear axle of the vehicle, again in dependency on the condition of the dynamic brake. In such braking systems, the mechanical brakes for the front axle are invariably actuated together with the dynamic brake and, under the aforementioned circumstances, simultaneously with the dynamic brake as well as with the mechanical brake or brakes for the rear axle.
A drawback of all presently known braking systems which employ mechanical and dynamic brakes is that, even though the mechanical brakes are actuated in automatic response to a reduction of dynamic braking torque, the combined braking action is reduced to a certain extent as soon as the mechanical brakes are actuated. Such reduction of braking force must be compensated for by the operator, i.e., the input element of the actuating means must be shifted to a different position. This is undesirable for a number of reasons, especially when one and the same person drives vehicles whose braking systems include dynamic brakes as well as vehicles with braking systems which do not embody one or more dynamic brakes. Such person requires a certain period of adjustment to operation of the braking system with or without a dynamic brake. This can result in accidents, especially during the just mentioned periods of adjustment.